If you will be using your mobile a lot to speak to the UK, either to make or receive calls, then consider buying a local sim card. Remember you will either need an unlocked phone or to buy a local phone as well. Making calls to the UK is in the order of 10p per minute, local calls and texts are a lot less. The difference between these costs and roaming charges soon pays back the approx £12 cost of the SIM. Even if you buy a cheap phone in India then you start pay back after about an hour of calls. Watch out for Indian roaming charges because mobile rates are by the state you are in. So if you move from Mumbai to, say Delhi, you get into roaming charges on your Indian number. Still, though, very much cheaper than roaming with your UK number! Check out the calling abroad rates on your UK mobile - they are probably a lot less than roaming abroad so forward your UK mobile to the Indian number. You can usually do this via an internet connection. Finally, remember to divert both your personal and business mobiles. If you are not going to buy an Indian SIM forward your personal mobile to your business phone so that calls into your personal phone cost you at your contract rate.
